(a) Technical Field
The present invention relates to a method for preparing a high-activity alloy catalyst for a fuel cell having a large active area by coating a carbon-supported platinum or platinum-transition metal catalyst with a conductive polymer such as polypyrrole (PPy) as a capping agent and performing heat treatment, thus increasing the degree of alloying and catalytic activity while preventing growth of particle size.
(b) Background Art
Fuel cells which convert chemical energy resulting from oxidation of fuel directly into electrical energy are spotlighted as a next-generation energy source. In particular, research for commercialization has been actively carried out in the auto industry for the purposes of improvement of fuel efficiency, reduction of emissions, environmental protection, or the like. In particular, extensive research has been focused on the catalysts for oxidation and reduction occurring on the electrodes of the fuel cells.
For commercialization of the polymer electrolyte membrane fuel cell (PEMFC), performance, cost, and durability issues have to be solved, all of which are closely related to the fuel cell catalyst. Since the PEMFC operates at low temperature, a precious metal such as platinum is used as a catalyst in order to increase the slow rate of oxygen reduction. However, the high cost and limited reserves of platinum delay its commercialization. For a fuel cell vehicle to be commercially viable, it is reported that the use of platinum should be reduced below 0.2 g per kW. However, theoretically, voltage loss occurs as the supporting amount of platinum decreases (e.g., to 0.4 mg/cm2 or lower). Thus, there is a limitation on reducing the use of platinum in the platinum catalyst. Further, since the slow oxygen reduction leads to over-voltage in the cathode, alloy catalysts are studied to improve the reaction rate.
As an alloy catalyst for a fuel cell, Pt3M of an oriented face-centered cubic structure with M being a transition metal (e.g., Ti, V, Cr, Fe, Co, Ni, etc.) is studied actively. Carbon-supported Pt3M is typically prepared by depositing a metal precursor on a commercially available carbon-supported platinum catalyst and carrying out heat treatment. Usually, after adding a transition metal precursor to the carbon-supported platinum catalyst, heat treatment is carried out at 700-1200° C. using a gaseous reducing agent such as hydrogen. Although the heat treatment improves catalytic activity by increasing the degree of alloying, it is accompanied by increased particle size and decreased dispersity.
Accordingly, methods for preparing alloy catalysts without the heat treatment process are studied. For example, certain previous methods to prepare alloy catalysts below 200° C. used carbonyl complexes (e.g., as described by Hui et al. in J. Phys. Chem. 108 (2004) 11024-11034), while other previous methods used the microemulsion method (e.g., as described by Xiong et al. in Electrochim. Acta 50 (2005) 2323-2329). However, the general colloid method is difficult in control of the degree of alloying and the transition metal easily dissolves out under the fuel cell environment since its concentration on the surface is high. One additional method, such as that described by Watanabe et al. in Appl. Mater. Interfaces 2 (2010) 888-895, succeeded in preparing small alloy particles with desired composition by using nanocapsules. One drawback to this method, however, is that it is not easy to remove the oleic acid and oleylamine used to create the capsules and, as a result, they lead to reduced catalytic activity. As such, since the currently available methods for preparing alloy catalysts at low temperatures have many problems, heat treatment at high temperature is unavoidable.